Different plastics... for water bottles they use mostly polycarbonate plastics (with the #7 recycle symbol) to get the clear, hard, glassy look. The polycarbonates are the ones that leach BPA/ Bisphenol A, really bad stuff.

The food grade 5 gallon buckets are made of HDPE high density polyethylene, recycle symbol # 2. These are fine to use and don't leach chemicals.